The present invention combines a class of novel siloxane monomers which contain an aromatic ring and vinyl functionality, as disclosed and claimed in our earlier application, with hydrogel forming monomers to form a polymeric hydrogel of improved oxygen permeability and mechanical properties.
It is important and essential that the cornea have access to atmospheric oxygen in order that an oxygen-carbon dioxide exchange can occur. Put another way, without constant eye exposure to the atmosphere, a state of oxygen edema can occur within the eye, which is potentially capable of causing damage. Thus, hard contact lenses, while having many practical advantages, generally are not altogether satisfactory because they most often have poor oxygen permeability.
Recently soft contact lenses, including the hydrogel type, have captured a significant market share. However, soft contact lenses are also not without disadvantages. Soft contact lenses generally have excellent eye comfort. However, soft lenses also readily attract and accumulate foreign debris, necessitating frequent cleaning. In addition, mechanical properties are often poorer with soft lenses.
Hydrogels are usually defined as natural or synthetic polymeric systems that contain approximately from about 10% to about 90% water in an equilibrium state. In general the physical properties of hydrogels are determined to a large extent by their water content. Due to their excellent biocompatability there has been extensive interest in hydrogels as biomedical devices. Thus there have been investigations on the use of hydrogels as contact lenses, intrastromal implants, intraocular lenses, coatings on numerous devices, membranes of several types, tissue replacement, ureter prosthesis, breast augmentation, etc. To date, the most commercial success has been found in the field of ophthalmology, and most particularly, soft contact lenses.
Hydrogel type contact lenses have been known, since at least as early as Wichterle et al, U.S. Pat. No. 3,220,960 which discloses a hydrogel which involves a hydrated polymer of an hydroxyalkyl acrylate or methacrylate cross-linked with a corresponding diester. Such gels may contain from about 10% to about 90% by weight water, preferably from about 30% to about 50% by weight water. Of the monomers used to prepare such hydrogels, 2-hydroxyethyl methacrylate is most commonly used. The equilibrium water content of lightly cross-linked poly (2-hydroxyethyl methacrylate) is about 40%. These hydrogels are often referred to as low water content hydrogels. Low water content hydrogels generally do not have as high an oxygen permeability as higher water content gels.
Another commonly used hydrogel system is based on copolymers of vinyl pyrrolidone and methyl methacrylate. The equilibrium water content of these hydrogels can vary widely as a function of the ratio of vinyl pyrrolidone to methyl methacrylate. However, most hydrogels of commercial interest have a water content in the 70% to 80% by weight range. These hydrogels are often referred to as high water content hydrogels.
As a general rule the low water content hydrogels have acceptable mechanical properties for application as a soft contact lens. However, they do not have acceptable oxygen permeability (DK) to be used as an extended wear contact lens. Also, as a general rule the high water content hydrogels appear to have acceptable oxygen permeability for application as an extended wear contact lens but have poor mechanical properties, i.e., are not easily formed into stable lenses, are tearable, sometimes lack visual acuity, and are easily damaged.
For many other biomedical applications it is also apparent that the utility of hydrogels have been limited by a lack of suitable mechanical properties. Accordingly, there is a real and continuing need to develop hydrogels which have improved mechanical properties and maintain the other desirable features of hydrogels such as biocompatibility, softness, transparency, and permeability to oxygen and other metabolites. The primary object of this invention is to fill this need.
It is more specific objective of the present invention to prepare hydrogel type contact lenses of improved mechanical properties which have good optical properties and acceptable oxygen permeability.
It is another specific objective of this invention to develop a hydrogel, which even at the lower water content levels, i.e. 40% to 50%, has good oxygen permeability.
These more specific objectives are achieved by the copolymerization of a hydrogel forming monomer (or monomers) and a novel class of siloxane monomers characterized by containing both an aromatic ring functionality and vinyl functionality at certain stereo-directing positions, which can be employed as a monomer for preparing copolymers useful as materials to form making a wide variety of types of biomedical products including soft, hydrogel type lenses, and ocular implants.
The method and means of accomplishing each of the above objectives, as well as others, will become apparent from the detailed description of the invention which will follow hereinafter.